Snow forecasting

Hydro-meteorological Prediction CenterAnother inch with the vort!

Snow Forecasting

• Things to think about when forecastingsnow and snowfall amounts

• How to forecast precipitation type• snowfall accumulations• A few empirical forecast techniques• Synoptic and mesoscale aspects of heavy

snow• Case studies

Forecasting snow requires• knowledge of the numerical models

– must resolve which model has best storm track

• knowledge of whether the pattern the model isforecasting favors a major snowstorm or a minorone.

• an assessment of whether the model is handlingthe mesoscale structure correctly.

• knowledge of the model low-level temperaturebiases.– For example, the models often warm the low level

temps too quickly across northern Maine

To forecast snowfall amounts

1 -- Need to forecast liquid equivalent (qpf)2 -- Determine rain/snow line, precipitation

type3 -- Then determine whether surface

temperature will allow snow to accumulate

4 -- Finally, predict snow to liquid equivalent ratio

The physical reasons thatdetermine the amount of snowthat falls over any location are

• The vertical transport of moisture into the system– vertical motion and moisture

• The efficiency of the precipitation processes(cloud physics)

• Size of the area of precipitating clouds• Propagation, are new snow producing clouds

developing upstream

Precipitation type• is dependent on the vertical temperature

structure– mechanisms that can change the vertical

structure include:• evaporation• melting• thermal advection• vertical motion• solar radiation (especially during spring)

• Is dependent cloud physics (freezing rain vssnow)

Traditional ways to forecastprecipitation type

• 1000-500 thickness– will not resolve thin warm layers

– warm boundary layer temperature or a warm layer abovethe surface

• 1000-850 and 850-700 mb partial thicknessmethods– better, but still may miss a very thin warm layer

• soundings and forecast soundings– the best method

1000-500 mb thickness

559554

549

544

539

534

529

524

519

5141000

-500

TH

ICK

NE

SS (d

m)

6 12 18 24 30 36 42 605448 72 7866STATION ELEVATION (HUNDREDS OF FEET)

50 % values of 1000-500 mb thickness as a function of station elevation

Strong marine influence

The critical thickness varies with elevation and with the dominantweather regime (stability of the airmass) that affects the station

Figure adapted from Glahn et al., 1975

THE 50% CRITICAL THICKNESS FORMOUNTAINS OF UTAH

1000-500 THICKNESS

540 550 560 570 580

ELEV

ATI

ON

IN T

HO

USA

ND

S O

F FT

4

5

6

7

8

9

10

11

12

13HAYDEN PEAK (580)

MT. TIMPANAGO (576)TWIN PEAK (575)

OQUIRRH MTNS (570)

SNOW BIRD (567)

BRYCE CANYON (558)

PARK CITY (TRAM) (565)

CEDAR CITY (549)

BLANDING (551)

LOGAN (545)

SILVER LAKE (563)

OGDEN AND PROVO (544)

MONTICELLO (554)

VERNAL (547)

SLC AIRPORT (543)

Partial Thickness

850-700 1000-850

<154

<154

<154

Precipitation Types

Significant UVV orlow-level coldadvection

Weak UVV and nearzero low-level coldadvection

>154

>154

>154

<129

129-131

>131

Thickness (dm)

Rain

>131

129-131

<129

Snow

Snow or sleet exceptmay be freezing rainnear 154, usually rainwith south winds inwarm sector

Snow, except sleetand/or freezing rain ispossible near 154

Sleet except may besnow near 154)

Sleet or snow except>152 usually freezingrain or drizzle, rain inwarm sector with southwinds

Freezing rain but maybe sleet near 154

Rain

Freezing rain, freezingdrizzle or sleet

Rain

Adapted from Cantin et al. 1990

Used for southeastern Canada

Rain

Freezing rain orfreezing drizzle

Precipitation type from soundingsThis is the best way to determine precipitation type

• Summary of important factors to look at onsounding– how warm is warm layer– what is the depth of the layer with wet bulb

temperatures above zero– wet bulb temperature of cold layer– depth of cold layer

The warm layer

• If Tw of warm layer exceeds 3 to 4o C, snow meltscompletely resulting in rain or freezing rain.

• If Tw is less than 1o C, only partial melting occursand snow will usually refreeze.

• If Tw is 1-3o C usually results in partial melting ofsnowflakes but then usually refreezes into sleet (ora mixture of sleet and freezing rain depending onthe depth of the warm layer.

From Stewart and King, 1987

Lower cold layer• If temperature is less than -10oC, and freezing

nuclei are sufficiently abundant and enough timeis spent in the cold layer, either snow or sleet canoccur.

• If cold layer is warmer than -8oC, droplets remainsuper cooled if the snow was completely melted(favors freezing rain).

• Depth of cold layer is not nearly as important asthe temperature of the cold layer.

From Stewart and King, 1987

A freezing rain soundingNote that temperature of the warm layer is above 4oC

0o

temperature

10o

An ice pellet soundingNote temperature of warm layer is 1-3oC

0o 10o

temperature

Unfortunately, a shallow warmlayer may show up on a forecastsounding. Use a combination offorecast soundings and MOSguidance to help predict the mostlikely precipitation type.

FREEZING RAIN OR SLEETTHE TAU TECHNIQUE - Cys et al., 1996

10

1

23

45

67

89

1000 2000 3000 4000 5000

FREEZING RAIN

ICEPELLETSM

EAN

LA

YER

TEM

P (C

O)

LAYER DEPTH (M)FROM SOUNDING

1. IDENTIFY DEPTH OF WARM LAYER (ABOVE 0OC)2. IDENTIFY THE MEAN TEMPERATURE OF THE WARM LAYER

3. THEN , FIND COORDINATE ON THE CHART ABOVE,THE YELLOW AREA USUALLY GIVES FREEZING RAINWHILE THE WHITE AREA GIVES SLEET

Freezing drizzle

• Bocchieri (1980) and Young (1978)found that 30% and 40% of freezingrain (usually drizzle) did not have alayer that was above freezing on thesounding.

• Huffman and Norman (1988) notesfor this type of freezing rain eventcloud top temperatures within thelow- level cloud deck should be inthe 0o to -10oC range and that thereshould be a pronounced dry layerjust above the cloud top. A typicalsounding for freezing drizzle isshown.

dry layer

0-20 -10900

800

700

600

500

Pres

sure

(mb)

Temperature (oC)

Sounding from Rapid City, SD at 00 UTC 12 March1976. Temperature (red line), dewpoint (dashed), frostpoint (blue dots).

dry layer

NCEP ETA PRECIPITATION TYPEALGORITHM

BLUE SHADED AREA IS WHERE MODEL SOUNDING SUGGESTS SNOW,VIOLET WHERE IT INDICATES SLEET AND RED FREEZING RAIN, DARKERBLUE LINES INDICATE Rh, WHITE LINES INDICATE VERTICAL MOTION.

Snowfall intensity• The rate that snow falls is a function of

– rate of growth of a single crystal• which peaks around -15oC

– and the number of crystals per unit volume,• the number can be increased

– by fragile crystals (dendrites and needles) fracturing– by ice splintering during riming– fragmentation of large super-cooled drops during freezing

When cloud top temperatures are -25oC or colder theconcentration of ice particles is usually sufficient to use up allthe condensate in stratiform and orographic clouds

ICE CRYSTALS GROW BY

250

200

150

100 50

0-2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22

150 sec100 sec

50 sec

TEMPERATURE (Co)

MA

SS (1

0-9g)

--Deposition, because esw>esi , vapor is transported from

--By collisions between super-cooled cloud drops and ice crystals

Experimentally determined variation of the mass of ice crystals growing bydiffusion of vapor in a water saturated environment, as a function of growth timeand temperature. (From Ryan et al., 1976; by courtesy of the AmericanMeteorological Society, and the authors.)

droplets to ice crystals

The variation of crystal habit with temperature and supersaturation

according to the experiments of Mason et al.

HO

LL

OW

PR

ISM

S

FA

ST G

RO

WIN

G D

END

RIT

ES

PLA

TES

Supe

rsat

urat

ion

with

res

pect

to ic

e (p

erce

nt)

0

10

20

30

40

50

Temperature-10 -20 -30 -40

Water-saturation

SEC

TOR

PL

AT

ES

SEC

TOR

PL

AT

ES

HOLLOW

NE

EDL

ES

SECTOR PLATES

THICK PLATES

PRISMS

SOLID PRISMSOLID VERY THICKPLATES

CUPSSOLID PRISMS

DEN

DR

ITES

SNOWFLAKE SIZE IS ALSO DEPENDENTON AGGREGATION

*MULTIPLE ICE PARTICLES FORM MAIN SNOWFLAKE

*AGGRAGATION PROCESS IS MAXIMIZED AS TEMPERATURE APPROACHES 0oC

70

60

5040

30

20

10

-15 -10 -5 0

DOMINANT CRYSTAL TYPEPLANAR DENDRITICRADIATING ASSEMBLAGE

Temperature (oC)Max

imum

snow

flake

dia

met

er (m

m)

Maximum observed snowflake diameters as a function of air temperaturefor two types of snowflake compositions. (From Rogers, 1974, 1974b)

WHY SHOULD I CARE ABOUT THE PHYSICALCHARACTERISTICS OF THE SNOWFLAKES

• The dominant crystal type may affect the snow to liquidequivalent ratio (how fluffy the snow is).– Unrimed Dendritic and plate crystals have a lacy structures that

usually produce the highest snow to liquid ratios (bestaccumulators)

– The make-up of the cloud may affect the snow to liquid ratio.When there is abundant liquid water in cloud causing crystals togrow by riming, snow to liquid ratios are lower (may be 10 to 1 orlower)

• Cloud physics effect how efficient the system is atproducing snowflakes. Dendrites crystals grow fastest.

• The size and composition of the snowflake may helpdetermine how quickly it sticks on the ground whentemperatures are marginal for snowfall accumulations.– Large aggragates may take longer to melt than smaller single

crystals.

Forecasting snow to liquid ratioSummary

• Warm ground and boundary layer temperatures can keep snow-waterratios down

• a warm layer that approaches zero oC also will usually keep the ratioslow.

• Storms having clouds with a large amounts of supercooled dropletswill not have as high a ratio as storms in which most crystal growth isby deposition.

• Soundings that are almost isothermal with a large portion of thesounding near zero oC will usually have a ratio of 8 or 10 to 1.

• Deep cold air promotes higher ratios but if the temperatures are toocold the crystal type may not be conducive to high ratios. .

• Storm tracks often provide keys to forecasting the snow to water ratio– tracks near oceans have more liquid water in clouds which usually

produces lower snow-liquid ratios

For exampleSouthern storm tracks typically are associated lower

snow to liquid ratios than clipper type systems

RATIO

8-1 SNOW-WATER

20-1 SNOW-W

ATER RATIO

6-1 SNOW-WATER RATIO,SNOW MAY MIX WITH RAINOR SLEET

AVERAGE SNOW-WATER RATIOS FORFOR SOUTHEASTERN WISCONSIN WITHVARIOUS STORM TRACKS

Adapted from Harms, 1970

1) Northern storm tracks that favorsnow crystal growth by depositionfavor high snow-water ratios.

2) When ice crystals grow by riming orcrystals colliding with supercooleddroplets, the snow-water ratios arelower.

3) Southern storm tracks and tracksthat tap moisture and warm air fromoceans rarely have snow to waterratios that are greater than 10-1 exceptwell west of the storm track. Look forlow ratios where precipitation becomesmixed with sleet.

Snow to liquid ratios vary significantly by geographic region.In Colorado the snow to liquid ratio is usually much higher

than 10 to 1 ( or snow density less than .10).

0

5

10

15

20

25

30

.00 .06 .12 .18 .24 .30SNOW DENSITY

Albany, NY hourlyobservations = 2328

PER

CEN

T O

BSE

RV

ATI

ON

S

Percent distribution of snowdensity based on 2328 hourlyobservations from 73 sitesnear Albany

0

2

4

6

8

10

12

14

16

SNOW DENSITY.00PE

RC

ENT

OB

SER

VA

TIO

NS

.03 .06 .09 .12 .15 .18

Lakewood, COhourly observations

= 62

Percent distribution of snow densitybased on 62 hourly observations from73 sites near Lakewood, CO

TemperaturesNear 32o

Adapted from Super and Holroyd, 1997

SNOW DENSITY AS A FUNCTION OF TEMPERATURE. NOTE THELOW CORRELATION SHOWN IN THE GRAPHS

ALB, NY AREA SNOW DENSITIES PLOTTEDAGAINST AIR TEMPERATURE. A LINEARREGRESSION LINE THAT BEST FITS THEDATA IS SHOWN IN BLACK. THE BLUELINE IS A 10 TO 1 SNOW TO LIQUID RATIO

SNOW DENSITIES AGAINST AIRTEMPERATURE FOR LAKEWOOD, CO. ALINEAR REGRESSION THAT BEST FITS THEDATA IS SHOWN IN BLACK. THE BLUE LINEIS A 10 TO 1 SNOW TO LIQUID RATIO

From Super and Holroyd, 1997

SNOW RATIO TABLE FOR THE EASTERN HALFOF COUNTRY (not mountain locations)

5 10 15 20 25

516

522

528

534

540

546

SNOW/LIQUID RATIO

SUR

FAC

E TO

500

MB

TH

ICK

NES

S

From Scofield and Spayd, 1984

At around 540 thicknessthe ratio was 10-1 orlower. At 528 the ratiowas around 17-1.However there wasconsiderable spread..

Other Tidbits about snow toliquid ratios

• The fluffiest snows (high snow to liquid ratios) usuallyoccur with light winds and temperatures near 15oF(-9.5oC).

• At colder temperatures crystal type and size change,– at very cold temperatures crystals tend to be smaller so

they pack closer together as they accumulate producingsnow that may have a ratio of 10 to 1 (sometimes evenlower.)

• A study by Mote (1991) found that ratios for Omaha,Nebraska averaged around 14 to 15 to 1 during the periodDec-Feb. and found that the highest ratios occurred withlighter snow events and the lower ratios with the veryheaviest snowfall. The heaviest storms had a 11-1 ratio.

Upper level aspects of major east coastsnowstorms (storms that produce a significant

sized area of 10” or greater snowfall• large increases in amplitudes between trough and

downstream ridge accompanied cyclogenesis• all cases exhibited decrease in half-wavelength indicative

of self-development process (increasing vorticity)• diffluence and negative tilt seen in nearly all cases• phasing of multiple vorticity maxima observed in about

half the cases• heavy snow usually falls as a vorticity max moves east-

northeast. heaviest snows fell north of the vorticity track• trough or upper-level cyclone was usually located over

eastern Canada– this provides confluence over Northeast allowing high pressure to

build From Kocin and Uccellini, 1990

THE IMPORTANCE OF THE EASTERN CANADAUPPER LOW AND CONFLUENCE FOR EAST

COAST SNOWSTORMS

HIGHLOW

SURFACE ISOBARSJET STREAK

500 MB HEIGHTS

THE UPPER LOW NEAR THE MARITIMESHELPS TO HOLD THE UPPER LEVELRIDGE AXIS NEAR THE GREAT LAKESREGION. THIS CONFIGURATION HOLDSCONFLUENT FLOW OVER THENORTHEAST AND LOCKS THE SURFACEHIGH OVER THE NORTHEAST.

THE TRANSVERSE CIRCULATIONASSOCIATED WITH THE ENTRANCEREGION OF THE JET STREAK KEEPS LOWLEVEL NORTHERLY FLOW ALONG THEEAST COAST AND PROMOTES DAMMING

SHIFT THIS PATTERN TO THE WEST ANDTHE SAME PATTERN IS FAVORABLE FORHEAVY SNOW OVER THE UPPERMIDWEST.

From Kocin and Uccellini, 1990

Cold air damming and coastal frontogenesis• Factors that favor cold air damming

– a cold surface high passes north of the Mid Atlantic States andNew England

– cold air supplied by the surface high is channeled southward alongthe east slopes of the Appalachians.

– A ridge of high pressure develops between the mountains and theocean. This helps to keep a northerly component to the low levelwinds over the land.

– Easterly or northeasterly low level winds over the ocean and themore northerly winds over the land tighten the thermal gradientnear the coast.

– When arctic air is present, the models often have a hard timeholding onto cold enough low level temps when the surface high isstill over New England or the Great Lakes regions.

18-20 MAR 1956

TROUGH“GREENLAND

BLOCK”CONFLUENCE

14-15 FEB 195818-21 MAR 19582-5 MAR 1960

10-13 DEC 1960

2-5 FEB 196111-14 JAN 196429-31 JAN 196623-25 JAN 19665-7 FEB 19678-10 FEB 1969

EASTERN CANADIAN

22-28 FEB 196925-28 DEC 196918-20 FEB 197219-21 JAN 19785-7 FEB 1978

18-20 FEB 19795-7 APR 1982

10-12 FEB 1983

YES

YESYESYESYESYESYES

YESYESYESYESYESYESYESYESYESYESYES

YES

YES

YESYESYESYES

YESYES

NO

YESYESYES

YESYESYESYESYESYESYES

YES

NO

NO

NONO

NO

NO

YES

YES

18-20 JAN 1961

YESYES

YES

NO

NONONONONO

YESYESYES

YES

YES

Table from Kocin and Uccellini (1990)

THE SELF-DEVELOPMENTPROCESS

NOTE THE SHORTENING OF THE HALF-WAVELENGTH BETWEEN THE TROUGHAND DOWNSTREAM RIDGE AXIS. THIS HELPS TO STRENGTHEN THE VORTICITY,VORTICITY ADVECTION AND THE UPPER LEVEL DIVERGENCE. THE SURFACELOW DEEPENS, PRODUCING INCREASED WARM ADVECTION WHICH BUILDS THESHORTWAVE RIDGE AHEAD OF THE TROUGH. THIS INCREASES THE AMPLITUDEOF THE SYSTEM.

JET

JET

JET

JET

From Kocin and Uccellini, 1990

The most common upper level jet patternwith snowstorms that produce a large

area of 10”+.

From Kocin and Uccellini,1990

The lower branch of the directioncirculation associated with thenorthern jet streak helps to providean northerly component to the lowlevel ageostrophic winds

The lower branch of the indirectcirculation supplies a southerlycomponent that helps enhance thelow level jet.

The two branches act together toenhance low-level frontogenesis andupper level divergence

SNOW AND THE SURFACE LOW,THE HEAVIEST SNOW FALLS

• AROUND 2.5 DEGREES LATITUDE (150 NM) TOTHE LEFT OF THE LOW’S TRACK.

• ABOUT 5 DEGREES LATITUDE (300 NM) INADVANCE OF THE LOW.

• AS THE CYCLONE DEEPENS.• HEAVY SNOW ENDS WHEN LOW BECOMES

VERTICAL AND START TO FILL.• HEAVY SNOW WAS USUALLY ASSOCIATED WITH

LOWS THAT WERE TRACKING TO THENORTHEAST FROM GOREE AND YOUNKIN, 1966

(OVER THE CENTRAL AND EASTERN U.S.)

850 LOWS AND HEAVY SNOW OVERCENTRAL AND EASTERN U.S.

• THE MEAN CIRCULATION INCREASEDSIGNIFICANTLY DURING THE 12 HR PERIOD OFHEAVY SNOW

• THE HIGHEST PROBABILITY OF HEAVY SNOWLIES APPROXIMATELY 90 NM TO THE LEFT OFTHE 850 LOW TRACK

• THE -5OC ISOTHERM NEARLY BISECTS THEHEAVY SNOW

• IN THE FRONT QUADRANTS OF THE STORMLITTLE WARMING TAKES PLACE– STRONG VERTICAL MOTION IS TAKING PLACE.

FROM BROWNE AND YOUNKIN, 1970

500 MB HEIGHTS AND 1000-500 MB THICKNESSTHE HEAVIEST SNOW (CENTRAL AND EASTERN U.S.)

DURING THE NEXT 12 HRS USUALLY OCCURS

• ABOUT 2.5 DEGREES LATITUDE (150 nm) TO THELEFT OF THE 500 MB VORT TRACK.

• ABOUT 6.5 TO 7 DEGREES DOWNSTREAM FROMTHE VORT.

• ALONG THE PATH OF THE 500 LOW, SLIGHTLYDOWNSTREAM OF WHERE THE CONTOURSCHANGE FROM CYCLONIC TO ANTICYCLONIC

• WITHIN THE 531-537 1000-500 MB THICKNESSCHANNEL, NEAR THE THICKNESS RIDGE

FROM GOREE AND YOUNKIN, 1966

COOK METHOD• AVERAGE SNOWFALL IN 24 HOURS WILL BE

ABOUT HALF THE WARM ADVECTION IN Co AT200 MB– PROVIDING WARM ADVECTION IS PRESENT AT 700 MB– IF COLD ADVECTION IS TAKING PLACE AT 700 MB,

THEN THE SNOWFALL WILL BE ABOUT A QUARTER OFTHE OF THE WARM ADVECTION

– 200 MB FLOW SHOULD NOT BE STRONGLY CONFLUENT

THE METHOD PROBABLY WORKS BECAUSE THE WARMADVECTION IS A MEASURE OF THE STRENGTH OF THETROPOSPHERIC FOLD AND THE POTENTIAL OF THEASSOCIATED CYCLONE TO DEVELOP.

Garcia techniqueFrom Cope (1996)

• Developed to forecast maximum snowfall potential• Use model forecasts of the isentropic surface located

about midway between the 700 mb and 750 mb pressuresurfaces.

• Average the mixing ratio directly over the area with thehighest “effective” value upstream.

• Use 6 hourly forecasts and total the average mixing ratio(g/kg) for each period.

Technique is very subjective and does not include an assessment of thevertical motion. Unfortunately, the amount of snow that falls is verydependent on the vertical motion!

Garcia method (continued)

PA

NY

OH

WV1

2 4 5

1312

11

63 7 8 9

NGM 6-H FORECAST ON THE 292OK ISENTROPICSURFACE, VALID 0600 UTC 8 JAN 1996. HEAVYBLUE LINES ARE MIXING RATIO (g/kg), DASHEDLINES ARE PRESSURE (TENS OF MB) AND WINDBARBS ARE IN KNOTS

WHAT IS THE EFFECTIVE MIXING RATIO FOR CENTRAL PA?

THE METHOD GIVES YOU A FEELFOR THE MAX SNOWFALLPOTENTIAL OF THE STORM BUTDOES NOT TELL YOU WHERETHE MESOSCALE BAND OFHEAVIEST SNOW WILL FALL.

FIGURE ADAPTED FROM COPE 1996

Magic ChartTHE MAXIMUM SNOWFALL IS EQUAL TO 2X(THE MAXVERTICAL DISPLACEMENT ON THE MAGIC CHART

THE TECHNIQUE CALLS FOR THE MAX SNOWFALL ALONGTHE -3o TO -5oc

NEED TO MAKE SURE THERE IS NO WARM LAYER

NOT MY FAVORITE TECHNIQUE

+14+12

10

0oC

-6oC

12 INCHES FELL IN CENTRALILLINOIS

From Chaston, 1989

Remember, snow usually occursin mesoscale bands.

• frontogenetic forcing along a boundary• a convergence zone, i.e. the Puget Sound convergence

zone.• By convective plumes induced to the lee of large open

expanses of water• upper-level jet streaks• terrain• gravity waves• conditional symmetric instability?

And can be focused by a variety of factors, for example:

Synoptic, mesoscale and local effects need to be considered whenforecasting snow

CONDITIONAL SYMMETRICINSTABILITY

• UNDER LARGE VERTICAL WIND SHEAR– THIS WILL CAUSE M.G SURFACES TO BE RELATIVELY

HORIZONTAL

• LARGE ANTICYCLONIC WIND SHEAR– ABSOLUTE VORTICITY WILL APPROACH ZERO AT THE

LEVEL WHERE CSI IS OCCURRING– THIS PRODUCES WEAK INERTIAL STABILITY

• LOW STATIC STABILITY– ISENTROPIC SURFACE AND MORE IMPORTANTLY THE

EQUIVALENT POTENTIAL TEMPERATURE (1e )SURFACE BECOMES ALMOST VERTICAL

IS FAVORED UNDER THE FOLLOWING CONDITIONS

From Bluestein (1986)

When trying to forecast CSI, make sure thecross section is normal to the 850-300 mb

thickness

The 850-300 . mb thickness for 00 UCT Jan 1991. Line A-Bdepicts the position of the cross section shown on next 2screens. Den=Denver

A

B

FROM MOORE AND LAMBERT, 1993

WHEN LOOKING FOR CSI ON A VERTICAL CROSS SECTION1) THE AIRMASS NEEDS TO BE SATURATED (>80% RELATIVE HUMIDITY).

2) CSI IS PRESENT WHEN THE SLOPE OF THE 1e IS STEEPER THAN THESLOPE OF THE MOMENTUM SURFACE.

3) THE LIGHT GREEN SHADING INDICATES AREAS OF CSI.

320

308

316

312

324

900

500

150

700

300

DEN

A B

34 101

44 109

20 40 60 80 100 120

328332

ABSOLUTE GEOSTROPHIC MOMENTUM

1111e

PRE

SSU

RE

ADAPTED FROM MOORE AND LAMBERT, 1993

CAN ALSO USE EQUIVALENT POTENTIALVORTICITY (EPV) TO FIND CSI

0.4

0.2

0.4

0.3

0.1 0.2

0.3

0.3

0.30.40.51.2.3.4.

5.

4.4.7.6. 8. 9. 9. 8. 7. 6.

4.

4.3.2.

1.

0.3

0. 0.

0. -0.1

DEN

900

150

200

800

700

500

300

44 109 34 101

PRE

SSU

RE

NEGATIVE VALUES DENOTE CSI (SHADED AREA)

AIRMASS MUST BE SATURATED

FROM MOORE AND LAMBERT, 1993

CSI and heavy snow• Numerous researchers have noted heavy snow

associated with CSI– have noted bands of heavy snow with CSI

• unfortunately, there is no good way to predict exactly wherethe bands will set up.

• McCann (1996) studied 14 cases of 10 inches ormore of snow east of the Rocky Mountains.– He noted that the tighter the horizontal temperature

gradient the better the chances for slantwise convection.– CSI may be common in cases with a strong horizontal

temperature gradient and CSI.

REMEMBER WHEN 1e DECREASES WITH HEIGHT, THE AIRMASS ISCONDITIONALLY UNSTABLE AND WILL PRODUCE UPRIGHT CONVECTION IFTHE AIRMASS IS SATURATED AND THE PARCEL IS LIFTED

References• Bocchieri, J. R., 1980: The objective use of upper air soundings to specify precipitation

type. Mon. Wea. Rev. 108, 596-603.• Browne, R. F. and R. J Younkin, 1970: Some relationships between 850-millibar lows

and heavy snow over the Central and Eastern United States, Mon. Wea. Rev., 98• Chaston, P.R., 1989: The Magic Chart for forecasting snow amounts. National

Weather Digest, 14, 20-22.• Cook, B. J., 1966: The Lubbock Snowstorm of February 20, 1961. U.S. Dept. of

Commerce, ESSA, Weather Bureau Southern Region, Tech. Memorandum No. 12. 10pp.

• Doesken, N.J. and A. Judson, 1996: The Snow Booklet, A guide to the Science.Climatology and Measurement of Snow in the United States. Colorado Climate Center,Colorado State University. 84 pp.

• Garcia, C. Jr., 1994: Forecasting snowfall using mixing ratios on an isentropic surface.NOAA Tech. Memo., NWS CR-105, U.S. Dept. of Commerce/NOAA/NWS. 31 pp.

• Goree, P.A. and R. J. Younkin, 1966: Synoptic Climatology of Heavy Snowstormsover the Central and Eastern United States, Mon. Wea. Rev., 94, 663-668.

• Harms, R. H., 1970: Snow Forecasting for Southeastern Wisconsin. NOAA TechnicalMemorandum NWSTM CR-38, U. S. Dept. of Commerce, NOAA, NWS, 17 pp.

• Huffman, G.J. and G. A. Norman, Jr., 1988: The Supercooled Warm Rain Process andthe Specification of Freezing Precipitation. Mon. Wea. Rev., 116, 2172-2182.

References Continued• Kocin, P. J. and L. W. Uccellini, 1990: Snowstorms along the Northeastern United

States Coast, 1955 to 1985. American Meteorological Society, MeteorologicalMonograph No. 44, 280 pp.

• Moore, J. T. and T. E. Lambert, 1993: the use of equivalent potential vorticity todiagnose regions of conditional symmetric instability. Wea. and Forecasting, 7 , 430-439.

• Mote, T.L., 1991: A statistical investigation of atmospheric thermodynamics andkinematics associated with intensity of snowfall at Omaha, Nebraska. Masters Thesis,University of Nebraska

• Ryan, B. F. , E. R. Wiehart and D. E. Shaw, 1976: The growth rates and densities of icecrystals between -3oC and -21oC. J. Atmos. Sci., 33, 842-850.

• Scofield, R. A. and L. E. Spayd, 1984: A technique that uses satellite, radar, andconventional data for analyzing and short-range forecasting of precipitation fromextratropical cyclones. NOAA Technical Memorandum NESDIS 8, 51 pp.

• Super, A. B. and E. W. Holroyd III, 1997: Snow Accumululation Algorithm for theWSR-88D Radar: Second Annual Report. Bureau of Reclamation Report R-97-05,Denver, Co, June, 70 pp.

• Young, W. H., 1978: Freezing precipitation in the southeastern United States. M. S.thesis, Texas A7M University, 123 pp.